This application relates generally to communication systems, and, more particularly, to wireless communication systems.
Wireless communication systems include a network of devices for providing wireless connectivity to wireless-enabled devices including mobile units, smart phones, tablet devices, laptops, desktops, and other types of user equipment. Exemplary access devices include access points, base stations, base station routers, node-Bs (NBs, eNBs), femtocells, and the like. The access devices and the user equipment communicate over channels of the air interface. The transfer function of the radio propagation channels of the air interface can be estimated and used to demodulate the data symbols that have been transmitted over the air interface. Conventional radio communication systems, such as systems that operate according to the Long Term Evolution (LTE) of the standards and/or protocols defined by the Third Generation Partnership Project (3GPP) standards body, embed reference signals into symbols transmitted over the air interface to enable the receiver to estimate the transfer function and demodulate the data symbols that are transmitted concurrently with the reference signals.
The LTE architecture implements orthogonal frequency division multiplexing (OFDM). A typical OFDM system transmits data over a large number of sub-carriers that are separated by a frequency spacing selected so that the subcarriers are orthogonal to each other. The data is divided into several parallel data streams or channels and each stream is allocated to one of the sub-carriers. Each sub-carrier is modulated with a conventional modulation scheme such as quadrature amplitude modulation or phase-shift keying. In LTE, time-frequency resources of the air interface are divided into resource blocks for uplink and downlink transmissions. A pair of LTE resource blocks (RBs) has a duration of 1 ms and a bandwidth of 180 kHz, which can be subdivided in the time domain into 14 symbols and in the frequency domain into 12 subcarriers. The smallest unit of resource is thus a resource element (RE) that represents 1 symbol on 1 subcarrier. A pair of resource blocks can therefore include 168 resource elements, e.g. for uplink and downlink transmissions.
The resource elements of each resource block are separated into data symbols and reference symbols. The number of reference symbols available to demodulate the associated data symbols is determined by the number of radio channels of the air interface, which is in turn defined by the number of transmit and receive antennas. For example, in a legacy single-input-single-output (SISO) system that includes one antenna for transmission and one antenna for reception, a single reference symbol may be used to demodulate data transmitted over the single channel between the transmitter and receiver. In order to allow the receiver to identify the reference symbols for different channels, different reference symbols occupy different patterns of resource elements within each resource block. As LTE systems become increasingly complex and incorporate larger numbers of antennas and radio links with multi-hop and multi-cell transmissions, the number of required reference symbol patterns will increase. In some situations it may become difficult to find a suitable pattern of reference elements for a reference symbol that does not collide with any other pattern of reference elements allocated to a different reference symbol.